We have studied by electron microscopy, and electron microscopic denaturation mapping, the following topics. Replication. Replication of lambda during both early and late stages of replication. Effect of various mutations (effecting replication) on the replicating structure. Replication of 186 DNA. Physical Mapping of P2 DNA. Heteroduplex and denaturation mapping of various deletions, additions, substitutions and hybrids of P2 DNA. RNA. Preliminary denaturation maps of R17 and Q beta double stranded RNA. Attachment of phage tails to DNA. Prior crosslinking of many phages results, on lysis, in phage tails attached to DNA. This observation allows conclusions to be drawn concerning the structure of DNA within the phage.